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The most important primula species in production today
include English primrose or acaulis primula (Primula vulgaris, synonym P.
acaulis), polyanthus, polyantha primrose or hybrid primrose (P. xpolyantha),
fairy primrose or baby primrose (P. malacoides), German primrose or poison
primrose (P. obconica) and cowslip (P. veris). Chinese primrose (P. sinensis)
is also produced in limited numbers and some interest exists in producing
drumstick primrose (P. denticulata) as a container plant.

Primula is widely produced in Europe as a flowering potted
plant for the late winter or early spring market. In the United States, growing
conditions in the Pacific Northwest are particularly well-suited for producing
primula. For garden and bedding uses, English primrose and polyanthus are
considered hardy to Zone 5, while fairy primrose, German primrose and Chinese
primrose cannot withstand frost. Species that are more appropriate for garden plantings
include garden auricula (P. xpubescens, a cross between P. auricula and P.
hirsuta), cowslip, Julian primrose (P. juliae), siebold primrose (P. sieboldii)
and red-leaf primrose (P. rosea).

Cultivars

English Primrose.
The English primrose has 2- to 10-inch-long leaves in a compact rosette, as
suggested by the word acaulis, meaning “stemless.” The flowers
develop on individual, 2- to 8-inch-long pedicels from the center of the plant.
The native English primrose has pale yellow flowers. Through breeding efforts
and the development of F1 hybrids, cultivars of English primrose are now
available with flowers in many colors from white to purple. The flowers may
have a yellow or white eye and are sometimes fragrant.

English primrose has been extensively bred, and a large
number of cultivars are available. The Danova series was introduced in 1989 by
Dæhnfeldt Inc. (Odense, Denmark) and may be the most widely produced
primula cultivar today. The Danova series is intended for early-season
marketing in November and December in the Northern hemisphere from a June
seeding. The Danova series comes in more than 20 colors and includes several
cultivars with 2-toned flowers. Other early-season cultivars include the Lovely
and Pageant series, bred by the Sakata Seed Corporation (Yokohama, Japan), and
the Quantum series from Goldsmith Seeds Inc. (Gilroy, Calif.). The Lovely
series has smaller flowers than the Pageant series, although they both have a
compact growth habit suitable for 4-inch or smaller pots. Several red and pink
bicolors are included in the Pageant series. The Quantum series has uniform
germination rates and good postharvest quality.

The Dania (Dæhnfeldt Inc.), Finesse (Ernst Benary Seed
Growers Ltd., Hann. Muenden, Germany) and Gemini (Goldsmith Seeds Inc.) series
are suitable for mid-season marketing (January and February). Compared to the
Danova series, the Dania series has somewhat larger flowers but a more limited
color range. A unique feature of the Finesse series is the narrow silver or
gold border of the flower petals. The limited leaf growth of the Gemini series
makes it suitable for 4-inch pots or in combination with other plants in patio
planters or color bowls.

in 1995 as a complement to the Danova series to extend the
marketing period. The Joker series consists of both bicolored and clear
flowers, and the Paloma series is a leading cultivar in Europe.

Polyanthus.
Polyanthus is a hybrid, primarily between cowslip, oxlip (P. elatior), English
primrose and Julian primrose. The flowers of polyanthus develop in a cluster on
an extended main flower stalk or peduncle (4-6 inches). Sometimes this
inflorescence is referred to as the polyanthus-type in contrast to the
single-pedicel acaulis type. Production guidelines, flowering requirements and
plant development are similar for polyanthus and English primrose.

The flower colors of polyanthus are dominated by yellows and
reds but cultivars with white, purple, bronze or gold flowers, with or without
a yellow or white eye, are available. For years, the most important polyanthus
cultivar has been the Pacific Giant series. The Sakata Seed Corporation has
been maintaining and developing the Pacific Giant series since 1968. This
vigorous-growing series with long peduncles has large clusters of flowers. The
newer Concorde (Dæhnfeldt Inc.), Hercules (Royal Sluis Ornamentals) and
Rumba (Goldsmith Seeds Inc.) series have more compact but stronger peduncles
than Pacific Giant. The Hercules series is also known for its postharvest
ability to ship well.

Fairy Primrose. The
flowers of the fairy primrose are arranged in a loose cluster with 2-6
superimposed whorls of 4-6 flowers. The flowers come in white, pink, red, mauve
or lavender shades. The leaves are 1-2 inches in length with 2- to 3-inch-long
petioles. The primary use of the fairy primrose is as a flowering potted plant,
since sensitivity to frost limits bedding and landscape applications.

The Prima series (Dæhnfeldt Inc.) is the most commonly
grown fairy primrose. This series comes as a seed mix of pastel flower colors with
a large proportion of bicolored pink and rose flowers or segregated into
specific flower colors. The expected production time is 5-6 months, and the
growth habit is uniform and compact.

German Primrose. The
German primrose has leaves up to six inches long with petioles of 2-4 inches.
Flowers appear in loose, upright clusters on 6- to 7-inch-long peduncles.
Cultivars are available with flowers in many pleasing, soft pastel shades from
white to lilac, purple, pink and orange. Expected production time for German
primrose varies from 4-6 months. The leaves of some German primrose cultivars
produce the allergen primin, which may cause skin dermatitis. The risk for
allergic reactions has limited its use and production. It is advisable to
observe caution when producing, handling and marketing the German primrose and
limit exposure by wearing gloves and long sleeves.

The German primrose series Juno (S&G Flowers, Downers
Grove, Ill.) has been grown extensively in the United States. The Juno series
has a height of 12 inches, relatively small leaves and abundant flowering. The
recent development and introduction of cultivars that do not produce primin has
reduced the skin rash problem and renewed the interest in German primrose. The
first cultivars stated to lack primin were introduced in 1990 under the names
‘Freedom’ and ‘Beauty’. Some individuals, however,
developed a skin rash following direct contact with these cultivars. In 1995,
the Libre series (Goldsmith Seeds Inc.) was released as the first true
primin-free selection of the German primrose. The Libre series grows shorter
than the Juno series at 8-10 inches, is suitable for 4- to 6-inch pots and
comes in several flower colors, including white, pink, salmon, red and blue.
Schoneveld Twello b.v. (Twello, The Netherlands) recently released the
primin-free Twilly series Touch Me.

Chinese Primrose.
The leaves of the Chinese primrose have a rounder shape than the other species
discussed here, and the leaf margins are scalloped. Cold hardiness is limited
to Zones 8-10. The flowers of the Chinese primrose form clusters on 4- to
6-inch-long peduncles. Various flower colors are available, from white and
purple to pink. Pigments often color the roots and the lower sides of the
leaves red. The Fanfare series (Dæhnfeldt Inc.) is produced in 5-6 months
and has exceptionally good shelf life, with large flowers. Drumstick
Primrose. Selections of drumstick primrose
are suggested to survive temperatures down to 50° F and have potential for
marketing as a flowering potted perennial for Northern landscapes. Flowers
appear in a dense, globular cluster on a peduncle. The flower color is white or
various shades of purple.

Cowslip. The cowslip
has fragrant, bright yellow flowers on an extended peduncle and is used in
rock, alpine or other types of gardens. Flower colors other than yellow
(orange, apricot, crimson, light purple and white) are now offered. The ability
to withstand low temperatures varies for cowslip from Zone 3 (-40 to -30°
F) to Zone 8 (10-20° F).

Propagation

Primula is exclusively propagated by seed. Germination is
often erratic and low, but improved seed quality and close attention to
moisture and temperature conditions have improved germination uniformity and
rates. A well-drained peatlite medium works well for germination and early
seedling development. A medium with 60 percent fine peat, 25 percent perlite
and 15 percent vermiculite may be recommended for primula plug production. The
sowing medium should have a low nutrient content and a pH between 5.5 and 6.0.
A medium electrical conductivity (EC) less than 0.75 dS·m-1 is essential
as high soluble salt levels may interfere with germination.

Recommended germination temperature is 60° F for English
primrose, polyanthus and Chinese primrose; 60-65° F for the fairy primrose;
and 65-68° F for German primrose. For good germination, maintaining the
temperature below 70° F for English primrose and polyanthus is important.
Chilling the English primrose seed to 40-50° F for one or two weeks to
improve germination at 65° F has been suggested. The response to chilling
has been variable and may depend on the cultivar and seed quality. One breeding
goal has been to improve germination. The currently available cultivars have
faster and more uniform germination rates and do not appear to require or
benefit from a low-temperature seed treatment.

Several reports indicate that light is required for maximum
germination. In other studies, light was not necessary for germination but
useful for controlling the height of seedlings immediately after germination.
In addition to providing light, sufficient moisture appears crucial for good
germination and early seedling development. Covering the seeds with a layer of
vermiculite, perlite or any well-aerated medium can be used to improve the humidity
around the seeds. The layer must remain thin due to the potential light
requirement. The small seeds of fairy and German primrose especially benefit
from a thin cover to avoid desiccation. For English primrose and polyanthus, a
protective layer can be added later when the radicle emerges 7-10 days after
seeding.

Early plant development

The germination process requires 10-14 days. Following
germination, the temperature can continue at 60-65° F for English primrose,
polyanthus and fairy primrose, and 65-68° F for German primrose. After 6-8
weeks, seedlings should have two or three true leaves and are suitable for
transplanting.

A single seedling is planted in a 3- or 4-inch pot. For
larger pots, two or more seedlings are used. Similar to the germination medium,
the growing medium should be high in organic matter and well-drained at a pH of
5.5-6.0. The planting depth should be the same as in the seedling flat to avoid
crown rot and other diseases. Plants are spaced when leaves reach the edge of
the pot. Suitable final spacing for 4-inch pots is four pots per sq ft.

Flowering requirements

English Primrose and Polyanthus
style='font-weight:normal'>. Production guidelines for flower initiation in
English primrose and polyanthus include several weeks at 40-50° F. The
temperature is dropped when plants have developed a good root system and 6-10
leaves. To promote bud set, some growers increase the fertilizer rate from
60-200 ppm nitrogen and double the potassium rate in relation to nitrogen two
weeks prior to temperature drop. The use of calcium nitrate and potassium
nitrate with low-proportion ammonium nitrogen has worked well. When flower buds
are visible, the temperature can increase to between 50 and 55° F or remain
below 50° F.

Cooling plants for up to 10 weeks increases the quality of
English primrose and polyanthus by increasing flower number, maintaining
smaller leaf size and reducing the pedicel length. The lower temperature,
however, slows overall plant development. Flower initiation has been observed
in plants grown continuously at 60-68°F, and the newer cultivars do not
require a cold treatment.

Slower flower initiation at lower temperatures has been
confirmed in more recent studies with rapidly flowering . Flower initiation at
46° F was especially slow when combined with low light (2
mol·d-1m-2) and short days (eight hours). Estimated optimum temperature
for flower initiation was 55° F. At 75° F, the English primrose Dania
failed to initiate flower buds.

A higher night than day temperature (negative DIF) has been
suggested to hasten flower initiation in English primrose. Flower formation and
development was faster at the negative (54/70° F, day/night) than the
positive DIF (75/54° F) at the same average daily temperature. Especially
under short days (eight hours), negative DIF appears to promote faster bud
formation and flowering than constant or positive DIF.

Even though primula is considered a low-light crop, daily
irradiance affected flower initiation efficiency. From seeding to floral
initiation at 54° F and long days (more than 12 hours), time to initiation
decreased from 72-57 days for English primrose at 10 mol·d-1m-2 compared
to 2 mol·d-1m-2. The optimum daily irradiance for flower initiation was
estimated to be 11 mol·d-1m-2 or approximately 1,250 foot-candles (250 µmol·m-2s-1)
for 12 hours. High natural light conditions may warrant shading to prevent
sunscald and improve temperature control since plant quality of English
primrose decreases above 68° F. Maximum peak irradiance in primula
production should not exceed 3,000 foot-candles (600 µmol·m-2s-1).
For most rapid flower initiation, the optimum daily light integral appears to
increase from 11-13 mol·d-1m-2 with either an increasing or decreasing
temperature from 55° F.

In contrast to earlier studies and recommendations, long
days have been found more beneficial than short days (less than 12 hours) for
rapid flower initiation and development. Cultivars that initiate flowers
without a drop in temperature appear to also have altered response to day
length. Long days are more beneficial for flowering of cultivars that do not
require a low temperature exposure. When natural day lengths are less than 12
hours, extending the day to 16 hours or utilizing 4-hour night interruptions at
a minimum of 10 foot-candles (2 µmol·m-2s-1) is now recommended.

Unsightly long and large leaves have been correlated to long
days or night break treatments and therefore avoided in primula production.
However, temperature now appears to be more critical than day length for plant
morphology. In polyanthus, more and smaller leaves were recorded at 68° F
than at 50° F, although plant leaf areas remained similar at the two
temperatures. High fertilizer levels and nitrogen in ammonium form are also
factors likely to result in large plants and leaves. Á

Primula flower initiation probably does not relate to plant
leaf area, leaf number or plant maturity. In Dania, flower initiation was
identified in plants with 6-26 leaves. At 46° F, 10 mol·d-1m-2 and
11-hour day length, plants with six leaves initiated flowers in 72 days from
seeding. At 68° F, 2 mol·d-1m-2 and 8 hours day length, plants
initiated flowers in 138 days but had 26 leaves.

Fairy Primrose. A
drop in temperature for six weeks is recommended for flower bud initiation of
fairy primrose. The temperature is reduced to between 45 and 50° F when
plants have reached the desired marketable size. Following six weeks of reduced
temperatures, 57-65° F is recommended. Fairy primrose will initiate flowers
without a reduction in temperature, although the final plant and flower quality
may not be as high.

Flower bud formation in the fairy primrose
‘Prima’ was independent of day length when grown at 60 or 68°F.
The appearance of flower buds was 20 days earlier at 60° F than at the
higher temperature. The development of buds into flowers on the other hand, was
affected by day length and was more rapid with long (16 hours) than short (8
hours) days at either temperature.

German Primrose.
German primrose is commonly grown commercially at 65-68° F. Reducing the temperature
to induce flowers is not recommended; however, the temperature can be dropped
to lower than 65° F during the final stages of plant development to improve
plant quality, flower color and size.

Guidelines for producing German primrose usually do not
include specific recommendations for day length. A high light integral (minimum
10 mol·d-1m-2) may shorten the production time, although during the
summer, shading is required to control temperature and avoid burning of the
foliage. The response to day length is temperature-dependent. The German
primrose Libre had 0.08-inch-large flower buds 90 days from seeding when grown
at 60° F and 8- or 16-hour day length or under 16 hours at 68° F. Short
days (eight hours) at 68° F delayed bud appearance by two weeks and flowers
failed to develop within 145 days from seeding. In contrast, long days at
68° F resulted in flowering after 111 days. At 60° F, the primin-free
Libre flowered faster under long (122 days) rather than short day length (133
days).

Nutrition

Fertilizing should start as soon as the cotyledons begin to
develop, about two weeks from seeding. The initial fertilizer rate should be
low at levels of 60 ppm nitrogen and potassium. The rate can increase up to 200
ppm nitrogen immediately prior to transplanting. During the production phase,
fertilizer rates of 90-100 ppm nitrogen and potassium from a complete
fertilizer with micronutrients are suitable. Excessive nitrogen and fertilizer
easily result in plants with too much leaf growth. Primula is sensitive to high
soluble salt levels that may result in necrotic leaf margin burns. Avoid
nitrogen in ammonium form under growing conditions with low temperature and
irradiance. High ammonium nitrogen levels may result in plants with
disproportionately long leaves. Micronutrient deficiencies or toxicities are
common in primula production and a medium pH between 5.5 and 6.2 is essential
for micronutrient availability. Regular monitoring of pH, soluble salts and
nutrient balance, through soil tests, is highly recommended.

Nitrogen deficiency appears as chlorosis in newly developing
leaves and as a chlorosis and necrosis of older plant tissues. Premature
flowering may also occur under nitrogen deficiency. The initial symptoms of a
phosphorous deficiency are bronzing of older leaves, inward curling of younger
leaves and leaf tip necrosis. Similar to nitrogen deficiency, plants may
prematurely flower. Signs of low potassium are chlorotic lower leaves slowly
turning necrotic and curling of leaf margins. Some plants may also die
following unexpected and sudden wilting with potassium deficiency. Polyanthus
grown with low or no calcium had poor root growth and pale green foliage.
Magnesium deficiency appeared as interveinal chlorosis with tip and marginal
necrosis of older leaves.

Primula is highly sensitive to reduced iron availability,
and deficiency symptoms are common. Early symptoms of iron deficiency are
chlorotic new growth followed by completely bleached white tissue due to a lack
of chlorophyll. Iron uptake and availability are greatly restricted above pH 6.
A boron deficiency initially appears as a light green color later turning into
chlorosis of recently matured leaves. The leaves may get cupped or crinkled
with leaf edges turning downward. The veins become excessively prominent,
especially on the lower side of the leaf. If a boron deficiency is not
corrected, the apical growing point dies and the stem becomes hollow.

German primrose requires higher fertilizer levels than the
other primula species. Nitrogen rates of 250 ppm with equal or greater
potassium levels are recommended for German primrose. On the other hand, German
and fairy primrose are highly sensitive to elevated soluble salts, which
manifests as leaf edge necrosis. Leaching at regular intervals is recommended
to avoid salt buildup.

Irrigation

During germination and early seedling development, the
medium should never be allowed to dry out. Seedlings are, however, sensitive to
over-watering. When seedlings of English primrose and polyanthus have established
following transplant, they can be allowed to dry slightly before being watered
thoroughly again. Fairy and German primrose are more sensitive to moisture
stress, and the media should be kept continuously moist. Plants allowed to dry
or grow at uneven moisture readily develop brown, dried leaf edges. High salt
levels in the medium result in similar symptoms as water stress in fairy and
German primrose.

Diseases and insects

Primula is relatively pest-free. Aphids, thrips, whiteflies
and caterpillars are the most commonly encountered pests. The primin-free Libre
is reported to be more susceptible to thrips than primin-containing cultivars.
During germination and early seedling development, fungus gnats and shoreflies
may become a problem. Control of algae growth and the use of insecticides may
be required to restrict the fungus gnat populations during early stages of
plant growth.

Tomato spotted wilt virus (TSWV) and impatiens necrotic spot
virus (INSV) have been identified in primula. The symptoms include browning
along the veins of the leaves and yellow mottling. No chemical controls exist
for viral diseases. The best control of TSWV and INSV is to rogue infested
plants and eliminate thrips, the insect vector that spreads the virus.

Postharvest

Primula is marketed when the first 5-7 flowers have opened.
Proper temperatures for shipping and holding are 36-43° F, and maintaining
well-watered plants is vital for longevity. Primula is highly sensitive to
ethylene. A silver thiosulfate spray application at 65-165 ppm has successfully
improved the keeping quality of English primrose.

The home environment is often at low relative humidity and
higher temperatures than the preferred 60-65° F. Although the keeping
quality is expected to be limited under these conditions, a high-quality
primula should flower and remain attractive for 10-12 days. With proper care,
the German primrose is expected to continue flowering for 2-4 weeks in an
appropriate postharvest environment.